In image formation by electrophotography, an electrostatic latent image is formed over an electrostatic latent image bearer such as a photoconductive material, a charged toner is attached to the electrostatic latent image to form a toner image, and the toner image is transferred to and fixed over a recording medium as an output image. In recent years, there has been an acceleration in the changeover from monochrome to full-color in the electrophotographic copier or printer technologies, and the full-color market is in the expanding tendency.
In full-color image formation, all colors are reproduced by overlaying three color toners, namely yellow, magenta, and cyan toners, or four color toners additionally including a black toner. Hence, in order to obtain a clear full-color image excellent in color reproducibility, it is necessary to smooth the surface of a fixed toner image and reduce light scattering. For this reason, many conventional full-color copiers and the like have an image gloss level of from 10% to 50%, which is a middle to high gloss level.
Recently, there have been requests for prints to be made over colored or black sheets of paper, which are colored base materials, or over transparent films, which are transparent base materials. However, a favorable color development cannot be obtained with the conventional four color toners. Hence, it is proposed to newly use a white toner as the fifth color toner and form a white background image (see PTL 1).
A white toner is used as a white base over sheets of paper, which are black or colored base materials, or as a white base over films, which are transparent base materials. In this case, the white toner is required to have a hiding property. The hiding property is an ability to make it impossible to see through the white base at which the white toner is fixed. In order for a white color to be developed as a solid white color with only a fixed white toner, it is necessary for the fixed white toner to scatter and reflect all incident rays of light. If it allows transmission of only a slight bit of incident light, its hiding property lowers, leading to perception of blur.
Various proposals have been made so far to improve the hiding property (see e.g., PTL 2 and PTL 3).
However, there is a problem unique to the white toner, i.e., degradation of flowability of the toner caused by increasing the pigment concentration with a view to improving the hiding property. When the toner has a poor flowability, aggregates of several toner particles are generated in a toner bottle or in a developing device. These aggregates get stuck at a member called doctor blade configured to regulate a developer over a developer bearer in the developing device, leaving this region alone keeping the developer from being uplifted therefrom, to thereby produce an abnormal image including a white streak, which is a void in an image running in the longer direction of the image.
To this problem, a solution by means of the toner is a matter of course, but a measure by any other means than the toner is also necessary because the fundamental cause is the high pigment concentration in the white toner.